It is desirable to be able to adjust the cam phase (i.e. the angular relationship between the crankshaft and the camshaft) of engines during engine operation in order to vary the timing of the opening and closing of the inlet and/or exhaust valves to improve engine performance, emissions and/or fuel consumption. Various types of arrangements exist that are capable of achieving this adjustment. Such system are required to efficiently convert rotational movement from the crankshaft into rotational movement of the camshaft whilst allowing dislocation between these rotational movements in order to allow variation of the cam phase.
Current systems typically utilize hydraulic actuators using high pressure oil to enable relative angular displacement between drive and driven members (e.g. cam sprocket or pulley and camshaft) of the valve train. Such systems have difficulty operating at extremes of temperature, in particular during engine start up when the oil is cold, due to temperature related viscosity changes of the oil.
Whilst attempts have been made to design electrically actuated variable cam phase arrangement, such usually require a complex triple shaft arrangement such as planetary gears or a harmonic drive arrangement. The complex arrangement of such triple shaft systems can introduce controllability and reliability problems and generally suffer from high power consumption.
Examples of triple shaft gear systems suitable for use with a cam phaser comprise planetary gear systems (as shown in FIG. 1), with a sun gear 1 planetary gears 2 mounted on a planet carrier 3 and ring gear 4, or harmonic drive systems (as shown in FIG. 2), with a wave generator 1′, flex-spline 2′ and circular spline 3′.
Another form of triple member system equivalent to a planetary gear system comprises a magnetic gear having an outer ring comprising a plurality of magnets (equivalent to the ring gear), a concentric inner ring comprising a plurality of circumferentially spaced magnets (equivalent to a sun gear) and an intermediate ring or carrier member mounted in an annular space between said outer and inner rings and comprising a plurality of circumferentially spaced pole pieces (equivalent to planetary carrier upon which a plurality of planetary gears are mounted).
Typically, where such triple shaft gear systems are applied to a cam phaser of an engine, one shaft is connected to the cam sprocket or pulley to comprise a drive member (typically the planet carrier or circular spline) and a second shaft is connected to the camshaft (typically the ring gear or the flex-spline) to comprise a driven member, the remaining third shaft (typically the sun gear or wave generator) being connected to an actuator for varying the cam phase (i.e. the angular relationship between the drive and driven members).
A particular problem with such known triple shaft gear systems is that the position and/or angular velocity of the third shaft must be controlled at all times to prevent uncontrolled cam phase change. Hence the actuator, typically an electric motor, must be powered at all times, leading to increased power consumption.